The origin and mechanism of transfer of a new mobile genetic element SCCmec

我们测定了从美国芝加哥儿童医院分离的CA 05和8/6- 3 P以及从日本门诊病人分离的81/108三株MRSA中SCCmecs的全核苷酸序列。3株SCCmecs均携带2型ccr A、B基因和由IS 1272-ΔmecRI-mecA-IS 431组成的B类mec基因复合物。3个SCCmec的J区（SCCmec中除ccr基因复合体和mec基因复合体以外的区域）与先前报道的3个SCCmec（I、II和III型）不同，且不完全相同。因此，我们将CA 05的SCCmec定为IVa型，8/6- 3 P定为IVb型，81/108定为IVc型。通过PCR实验，发现三株甲氧西林敏感型菌株，S。hominis，S. arlettae和S. arincularis携带ccr基因。我们选择了一株S.与人型GIFU 12263进行同源性比较，测定了该菌株ccr基因的核苷酸序列，并鉴定了一个独特的结构，命名为SCC_<12263>。我们认为SCC_1<12263>是葡萄球菌盒式染色体mec（SCCmec）的一个活跃的原始移动的遗传元件。我们克隆了S.人GIFU 12263以及来自CA 05和81/108的ccrA 2和ccrB 2基因。发现这些orfs催化精确切除来自耐甲氧西林葡萄球菌I型和IV型的SCCmec，SCCmec可以从MRSA转导到MSSA。然而，SCCmec的接合转移尚未得到证实。

We have determined entire nucleotide sequences of SCCmecs in three MRSA strains, CA05 and 8/6-3P which were isolated from Chicago Children's Hospital, U.S.A, and 81/108 which was isolated form an outpatient in Japan. The SCCmecs of all three strains carried type-2 ccr A, B genes, and class B mec gene complex composed of IS1272-ΔmecRI-mecA-IS431. The J-regions of three SCCmecs(the region except ccr gene complex and mec gene complex in SCCmec) were not identical and differed from previously reported three SCCmecs(type-I, II, and III). Therefore, we have designated the SCCmec of CA05 as typeIVa, that of 8/6-3P as typeIVb, and 81/108 as typeIVc respectively. By PCR experiment, we found that three methicilin-susceptible type strains, S. hominis, S. arlettae, and S. arincularis, carried ccr genes. We selected a strain, S. hominis GIFU12263, determined the nucleotide sequence around ccr gene of the strain, and identified an unique structure designated SCC_<12263>. We consider that SCC_<12263> is an active primordial mobile genetic element for the Staphylococcal Cassette Chromosome mec (SCCmec). We have cloned ccrA1 and ccrB1 genes from S. hominis GIFU12263 as well as ccrA2 and ccrB2 genes from CA05 and 81/108. Those orfs were found to catalyze precise excision of the SCCmec from the methicillin-resistant staphylococci Type-I and Type-IV SCCmec could be transduced from MRSA into MSSA. However, conjugative transfer of SCCmec was not proved yet.